The use of ultraviolet (UV) curable coatings on polycarbonate substrates is desirable due to the advantage of being able to cure the coating to the substrate in a minimum amount of time without the need for heat which could destroy the substrate. To obtain superior adhesion between the UV coating and the substrate, the proper selection of monomers in the coating composition is critical.
The best coating-substrate adhesion is experienced when utilizing coatings containing so-called "interacting" monomers, that is, monomers which are reactive with the polycarbonate substrate. UV curable coatings which contain interacting monomers exhibit much better adhesion to polycarbonate substrates than UV curable coatings which contain non-reactive, or so-called "noninteracting" monomers.
Priola et al. suggest that the production of a haze, or opacity, upon curing indicates that the coating and the substrate have interacted to form a good, adhesive bond. They found a correlation between opacity and the use of the interacting monomers. Consequently, they suggest that to obtain good adhesion between the UV curable coating and the polycarbonate substrate one should select interacting monomers for use in one's UV curable coating composition. However, the very formation of the haze or opacity is a major problem associated with the use of interacting monomers in UV curable coatings compositions. The presence of a haze or opacity in the coating composition detracts from its visual appearance and appeal. Consequently, the dilemma becomes how to obtain good adhesion between UV coating compositions and polycarbonate substrates without producing the undesirable haze or opacity associated with the interacting monomers.